Continuous casting of steel and other metals is well known in the art. Typically, a nozzle (called a "subentry nozzle") is employed to deliver the molten metal from a tundish into a continuous casting mold below the surface of the body of molten metal adjacent the upper portion of the mold. Entry of the molten steel from the tundish into the upper end of the nozzle is controlled by a stopper-rod which seals and closes the entrance to the nozzle. Means are provided for moving the stopper-rod vertically. As the stopper-rod is moved upwardly away from the nozzle, the molten metal flows into the nozzle through the annular space formed between the stopper-rod and the upper end of the nozzle.
The nozzle typically has a flow rate substantially greater than the optimum casting rate of the apparatus, and the casting rate of the apparatus is controlled by regulating the distance of the stopper-rod from the upper end of the nozzle. The cross-sectional area of the nozzle in some instances approaches twice the cross-sectional area needed for the optimum casting rate. Large bore nozzles (80-90 mm) casting typically aluminum-killed steels have, however, experienced aluminum oxide accumulation under the stopper-rod which restricted and closed-off the flow of molten metal. The accumulation of aluminum oxide may be minimized by introducing an inert gas, such as argon or nitrogen, into the upper end of the nozzle through an orifice in the stopper-rod, but this technique fails to prevent aluminum oxide accumulation in the lower part of the nozzle where the molten metal flows out the nozzle into the mold. See U.S. Pat. Nos. 3,886,992, 3,888,294 and 3,935,895.
Various shrouds and nozzles have been proposed for retarding oxidation of the molten metal during casting. Illustrative of the art is believed to be the disclosures of U.S. Pat. Nos. 2,005,311, 2,503,819, 3,208,117, 3,439,735 3,608,621, 3,746,077, 3,886,992, 3,888,294, 3,908,735 and 3,935,895, French Pat. Nos. 1,542,950 and 1,586,666, and Dutch Pat. No. 228,418. The most pertinent of the prior disclosures is believed to be U.S. Pat. Nos. 3,886,992, 3,888,294, and 3,935,895, mentioned above, and U.S. Pat. Nos. 3,439,735, 3,451,594, and 3,608,621.
U.S. Pat. No. 3,451,594 is directed to a tundish nozzle which has a series of apertures about its lower extremity directed parallel to the nozzle opening. As an uncontained stream of molten steel passes between the bottom opening of the tundish nozzle and the mold cavity, inert gas is ejected from the apertures around the opening parallel to the molten stream to form a curtain completely enveloping the molten stream so as to prevent the formation of spinels, such as iron oxide-aluminum oxide.
U.S. Pat. No. 3,608,621 discloses a method for controlling the flow of molten metal from a tundish into a continuous casting mold by regulating a gas supply to the apparatus as a function of the level of molten metal in the casting mold. The apparatus has a casting tube mounted in the tundish and extending through the bottom of the tundish, with its upper, open end extending upwardly from the bottom of the tundish to a height above the normal level of molten metal in the tundish. A rising tube is concentrically positioned around the casting tube with its upper end extending above the upper end of the casting tube and its lower end having openings through which molten metal in the tundish can flow into the space between the tubes. The molten metal is caused to rise in the space between the tubes by a controlled supply of gas, such as argon, to the molten metal in the lower part of the space between the tubes to mix with the molten metal and cause the molten metal to rise up and overflow into the casting tube in a controlled flow.
U.S. Pat. No. 3,439,735 describes a stream protector which surrounds in non-contacting relation a free-falling stream of molten metal, for retarding the atmospheric contamination of the molten metal as it is teemed from a tundish to a mold. A pair of annular gas channels separated by baffles are placed at the upper extremity of the protector near the input port of the molten metal to project a cylindrical curtain of inert gas around the free-falling stream of molten metal. It is expected, however, that the lower portion of the protector is subject to metal oxide deposits as the inner surface of the protector becomes more distant from the gas channels.
The present invention overcomes these disadvantages and difficulties. It provides a nozzle for delivering molten metal from a tundish below the surface of a body of molten material in a continuous casting apparatus while inhibiting the formation of aluminum oxide and the like in the lower discharge portion of the nozzle.